Recent advances in the field of optics have led to the integration of optical elements into miniaturized optical assemblies as well as combinations of optical and electronic elements in miniaturized opto-electronic assemblies. Such assemblies include exposed assemblies as well as hermetically sealed optical and opto-electronic packages of various types. In most cases, mechanical and electronic elements are used for adjusting the positions, orientations and operation of the optical elements as well as for converting optical signals propagating in the form of beams into electronic signals and vice versa. More specifically, electro-mechanical actuators are used for adjusting and controlling the positions of optical elements, while the electronic elements are used for sensing position errors and for driving electro-mechanical actuators.
Whatever the function of the optical and opto-electronic assemblies, in most of them at least a portion of the optical, path of the beam or beams passes through free space rather than a waveguide. In other words, the beam or beams propagate through free space between waveguides or optical detection devices contained within the assembly. In some assemblies the beam or beams are generated by a source, e.g., a laser diode, and after out-coupling from the diode they propagate through free space to all the subsequent optical elements without ever being in-coupled into a waveguide. For example, in a laser pointing or aiming device, a beam is generated by a laser diode and propagates through free space to the collimating and focusing lenses as well as any deflectors (mirrors) and to the target outside the laser pointer.
For the above reasons, many of the elements contained in optical and/or opto-electronic assemblies perform the functions of beam steering or guiding systems. In particular, the beam guiding systems typically consist of optical elements such as mirrors, lenses, refractive elements and/or diffractive elements as well as optical component mounts and opto-mechanical components for positioning and supporting these optical elements. These elements are usually passive.
Optical and opto-electronic assemblies are frequently exposed to environments or mechanisms producing mechanical vibrations. Among the many mechanisms responsible for such vibrations one can mention microphone pickup, air disturbance, vibration coupling from other assemblies and/or external mechanical parts, mechanical shock, and slower thermal drifts. The vibrations translate to deflections of the beam because the optical elements of the beam guiding system move relative to each other. A number of even relatively small deflections occurring at several optical elements can add up to an unacceptably large total deflection of the beam. For example, in the case of a laser pointer, several small deflections in the beam guidance system can add up to a significant fraction of a degree total deflection, which is unacceptable to the user. Of course, in some cases deflection of the beam at just one optical element is sufficient to cause an unacceptable total deflection of the beam. Hence, it would clearly be advantageous for the beam guiding system to be immune to vibrations.
The prior art teaches a number of mechanisms developed for reducing or compensating vibrations in large-scale optical systems such as cameras. For example, U.S. Pat. No. 5,585,875 to Imafuji et al. discloses a camera having a vibration correction system that detects vibration of the camera caused, e.g., by hand tremor, and corrects for the vibration of an image in an image plane caused by vibration of the camera. Imafuji's system detects the vibration with the aid of vibration sensors, e.g., acceleration sensors. The reader will find additional teachings on vibration detection and compensation methods for cameras and optical imaging systems in the open literature, including U.S. Pat. Nos. 5,740,472; 5,682,556 and 5,335,032.
For further information about acceleration sensors, also called accelerometers, the reader is referred to U.S. Pat. No. 6,389,899 to Partridge et al. and Kevin E. Burcham et al., “Micromachined Silicon Cantilever Beam Accelerometer Incorporating an Integrated Optical Waveguide”, SPIE, Vol. 1793, (1992), pp. 12–18. For further information about measurement units incorporating acceleration sensors the reader is invited to review U.S. Pat. No. 6,456,939 to McCall et al.
Unfortunately, the above teachings cannot be used to resolve vibration-related problems in modern optical and opto-electronic assemblies for a number of reasons. First, the solutions implemented in cameras and other large optical devices including imaging systems simply do not scale to the miniature optical and opto-electronic assemblies. Second, the solutions implemented in cameras, which are a single integrated system with a well-defined end use, cannot be generalized to components used in laboratory environments where the end-use cannot be predicted. Third, the mechanisms used to compensate for vibrations in large-scale devices such as cameras include shutter timings, exposure controls and other controls that have no equivalents in optical and opto-electronic assemblies. Fourth, cameras are devices that accept light from an outside source, and may compensate for motion of the imaging device relative to the inertial coordinate frame by internal detection of vibration signals. In contrast, modern optical and opto-electronic assemblies are non-imaging devices with their own light sources, e.g., solid-state lasers.
Therefore, in response to vibration-related problems encountered in small-scale optical and opto-electronic assemblies, most prior art references teach to monitor output deflection of a beam 1 exiting from an optical or opto-electronic assembly 2 as shown in FIG. 1. For this purpose a beam portion 3 of beam 1 steered by elements 4 of assembly 2 is tapped with the aid of a beam splitting element 5. Beam portion 3 is delivered to an optical position sensor 6, which tracks the position at which beam portion 3 is incident and/or monitors its spot size. Further information about such systems can be found in S. Grafstrom, U. Harbarth, J. Kowalski, R. Neumann and S. Noehte, “Fast Laser Beam Position Control with Submicroradian Precision”, Optics Communications, Vol. 65, No. 2, 15 Jan. 1988.
Unfortunately, monitoring of vibration by tapping the output beam has many drawbacks. First and foremost, tapping introduces losses and potential for undesired back-reflections. Second, the introduction of tapping optics is not feasible and downright impossible in many systems due to dimensional constraints. Third, the tapping method typically monitors a total deflection of the beam at the output of the assembly and hence does not yield any information about the optical elements causing the deflection. Fourth, a monitoring method based on tapping is limited to observation of the position or vibration of an output beam relative to its local surroundings. In the case of a hand-held, or otherwise movable device, the output may move relative to a world (inertial) coordinate frame, and be undetectable by an internal tap.
The problems associated with vibrations are especially acute in systems employing optical component mounts for supporting the optical elements. In such systems the optical mounts transfer vibrations associated with their vibrational states to the optical elements mounted on the optical mounts.